Notice how both channels of IC205 share the existing capacitor. It may be the same case with the SY77 that I might have missed in the mess of crossing lines. Anyway...

Now ICs 202, 203, 204 and 209 and 268 are all M5238s!

Notice how there is one extra stage in there.
While the SY77 has one Sallen-Key module composed by IC11 to filter out at 12dB/Oct. unwanted high-content noise in the audio path, the 99 adds a second module in series for a steeper, 24dB/Oct. cut (ICs 203 and 268).
I´m guessing this extra filtering was meant as a precaution due to the much better response of the M5238 in the frequency extremes.

So, swapping the poor RC4558s for the fast M5238s may not be such a good idea. It could be that Yamaha´s engineers were being overzealous, if not a little paranoid. BUT to go to the extent of modifying the output stage to accomodate faster opamps is a sure sign an upgrade to the tone of the previous model was in their plans.

I´m therefore going to swap the RC4558s not with M5238s but with opamps designed specifically for Audio, the NE5532, which not by coincidence are my favourites. They are not as fast as the 5238, so they´ll be OK with the existing filtering in the SY77, but are Lightyears away from 4558, with a "musically informative" midrange to die for.
I have done a similar experience swapping these two ICs on a Roland D550. Anyone interested please search the forum for the relevant thread.

Thanks for the very interesting post. Deeper than my knowledge of electronics goes; but I own multiple SY77s and 99s; and know of several exquisite service engineers here in Ireland so will read your posts over a few times to gain clarity on what you've done, and download the schematics with a view to perhaps having one of my SY77's modified.

I have to admit, I do notice a slight difference in the tone of the 77 and 99 with the same programs loaded, but I wouldn't have regarded one as superior to the other; though I make no claim to have listened in any controlled way to identify differences. But - my favourite SY77 preset - Bah Mallet - does sound a tad sharper on the SY99 - but of course I slightly prefer it on the SY77 because I bought that first - so even if slightly less well defined - it's still "the original"

I´ll have to look at the schematics but it appears the output routing is also done in hardware via shorting jacks. This means, yet to be confirmed, that just sending output 1 to the audio interface (for a combined outputs1+2) and alternating between volume sliders to select outputs, the signal still goes through the output 1 stages, unless a pair of dummy plugs is used on output 2. OK this sounds confusing.

Another thing that I may have not foreseen is that every stage, including the Sallen-Key filters, is unity-gain.
Just like the noise figures don´t change unless some gain is added, could it be that an opamp "signature" can only be noticed if some gain is added? Wouldn´t that mean a 741 opamp buffer could be placed after the output of a really good preamp without any perceived sonic difference? I highly doubt that

Just checked: slew rate remains constant regardless of gain. The RC4558 has a slew rate of 1.7V/uSec. Compare that to the slew rate of the NE5532 at 9V/uSec. It´s impossible not to see an improvement. However, the M5238 reigns supreme at 20V/uSec...

Damn this experiment started on the wrong foot by not bringing all the sockets!
I´ll have to open up the unit, put the 4558s back in place, record an audio file, open up the unit, replace all 4558s with 5532s, and more importantly, replace the display! Then record an audio file and compare.

I´ll leave it for now as I suddenly became lazy just for thinking about it. Why didn´t I try with the TG77 instead? Duh...

This topic has already been debated at Gearslutz, two years ago. Hmm interesting forum.

No one apparently tried replacing the DAC. According to the datasheet, the 18-bit PCM61 is a direct substitute for the 16-bit PCM56.
Will it make a worthy improvement, or a big noisy mess? There´s only one way to find out...

There were no more 5532 left at my local suplier and couldn´t wait for the 5238 to arrive from Asia, but there were enough TL072s. So, all the opamps being 072s, except for the headphone amp, now we´re takling. A "dawn and day" difference in clarity and definition. All for less than 8 euros It costs less than one of those recent "boutique" opamps which miss the point IMHO. This SY77 still sounds early 90s, but much better. I don´t know what Yamaha had in mind with the 4558s, but I don´t think it was preplanned obsolescense. It were probably the bean counters that forced the team of enginneers to choose them. The 4558 isn´t really up to the task. The sound engine can do much better.
Bye.

Well in that photo I posted in post#7 it´s every eight-pin chip except the top one, which is closer to the output jacks. That one is the phones amp and it´s a good chip, no need to change it. So the remaining eight pin chips are three M5238s and six RC4558s. I replaced all with TL072s. They sounded good to me. They´re cheap and easy to get. Like I said, nine 072s plus nine sockets cost less than eight euros. No big deal if one doesn´t like the change, just leave the sockets in place and put the old chips back.